As subsea structures get larger and more complex, landing speed of subsea structures becomes an increasingly important topic. Cranemaster® has been used extensively to reduce motion for subsea lifting operations. Boom tip movements are picked up by the stroke of the Cranemaster® unit and thereby limiting the load movements.

Subsea landing of structure using Cranemaster®

Source: Subsea 7, Chevron

Notice the stroke.

Notice the complete lack of vertical motion on the object.

Key considerations

Cranemaster® will be more efficient for loads with high drag forces and large added mass such as manifolds, mud mats, protection covers and suction anchors. It will be less effective for objects with a small horizontal area such as spool systems.

Since the forces preventing the object from movements are dependent of velocity and acceleration, the performance of Cranemaster® will decrease with increased wave periods.

Cranemaster® will itself have a resonance frequency which may be close to the wave periods. This may cause increased motion and velocity, and pre-calculation and/or system simulations are therefore important especially for objects with low drag.

Tuning and performance

Cranemaster® should have a flat spring versus stroke curve to minimize spring forces. Dampening forces from Cranemaster® should be carefully controlled.

A reduction in landing speed from 0-95% can typically be obtained for a well-designed Cranemaster® setup. The efficiency of Cranemaster® is dependent on the average stiffness being less than the sum of forces acting on the load. As a consequence, Cranemaster® units used for passive heave compensation have large accumulators to reduce the average stiffness.

The performance of Cranemaster® will depend significantly on the hydrodynamic properties of the structure/object. The most important parameter is the vertical drag. A large drag area will give high reduction in landing speed.

Featured References – Subsea Landing

Customer Challenge: Deployment of a large mudmat (13,75 x 13,75 meter) at 1788 meter water depth in the Gulf of Mexico. For ease of operation, the mudmat is deployed horizontally. Considerations to [...]